1. Field of the Invention
Embodiments of the invention relate to systems and methods for recovering hydrocarbons from a reservoir. Embodiments disclosed herein are particularly useful, for example, in recovering heavy oil and bitumen.
2. Description of the Related Art
Grades of oil are generally separated into classes according to viscosity, density, and sulfur content. The higher the viscosity and density of the oil, the more difficult to produce the oil from reservoirs to the surface. In particular, extra heavy oil (EHO) and bitumen require production enhancement techniques for production. In the following description, the generic term “oil” may be used in reference to extra heavy oil and bitumen, but also applies to less viscous grades of oil.
A large portion of the world's potential oil reserves are in the form of extra heavy oil and bitumen, such as the Orinoco Belt in Venezuela and oil sands in Alberta, Canada. Currently, existing bitumen and extra heavy oil reservoirs are exploited using enhanced thermal recovery techniques or solvent based techniques resulting in efficiency of recovery in the range of between 20 and 25%. The most common thermal technique is steam injection in which heat enthalpy from the steam is transferred to the oil by condensation. The heating reduces the viscosity of the oil to allow gravity drainage and collection. Injection may be achieved by the well known cyclic steam simulation (CSS) and Steam Assisted Gravity Drainage (SAGD).
In SAGD, two primary wells are drilled into the reservoir: a production well and an injection well. The injection well is used to inject steam and various gases into the reservoir. Production of hydrocarbons though the production well results from the reduced viscosity and higher temperature and pressure resulting from the injections.
The costs and environmental impact of recovering the extra heavy oil and bitumen is an ongoing concern. The steam generators require significant amounts of fuel to produce sufficient amounts of steam to stimulate production. The ratio of steam to oil produced (“steam on oil ratio” or SOR) using current techniques ranges from 1.4 to 4. In other words, 1.4 to 4 gallons of water must be evaporated into steam for each gallon of oil produced from the reservoir. In addition to the fuel consumed, much of the water is lost to the reservoir. What water returns to the surface with the produced oil must be treated to remove contaminants, such as heavy metals and sulfur. The treatment of water further adds to the cost of production.
Greenhouse gas emissions from steam generation are also of concern. As an example, approximately 8,000 to 15,000 tons of carbon dioxide (CO2) may be generated daily to produce injection steam and produce 100,000 barrels of oil per day (BOPD) of bitumen. Reduction in CO2 and other greenhouse gas emissions would lessen the environmental impact from extra heavy oil and bitumen production.
Vapor extraction (VAPEX) is another technique for enhancing production of extra heavy oil and bitumen. The VAPEX process involves injecting a gaseous hydrocarbon solvent into the reservoir where it dissolves into the oil, thereby reducing viscosity and allowing drainage into a lower horizontal well for extraction. Typical hydrocarbon solvents include propane, butane, or CO2 with a carrier gas. Currently, VAPEX alone, without also heating the reservoir, produces small improvements in oil recovery. The hydrocarbon solvents are expensive and a large percentage is lost in the reservoir during production.